The media client must play videos by beginning with a random access point. Therefore in the traditional multicast video service, in order to be adapted for playing videos when different media clients joined in the multicast group at different time, the multicast server generally sends random access point in a fixed period. And the media client obtains the random access point through the multicast address. In order to make the client newly joined in the multicast group play pictures rapidly, the period is generally very short. The random access point of the video adopts the intra-frame coding; and the coding time and frame size after the coding are far greater than the normal frame which adopts the inter-frame coding. Consequentially, the time interval between the video streaming random access points is in inverse proportion to the coding, decoding and transmission efficiencies.
The technical solution of the Rapid Acquisition of Multicast Realtime Transport Protocol Sessions (RAMS) of Internet Engineering Task Force (IETF) provides a method of rapid acquisition of media client in a Realtime Transport Protocol (RTP) multicast session. Compared with the traditional multicast video service method, the method using the RAMS may reduce the delay time of the media client before joining in the RTP multicast session. In the streaming media multicast service of the multicast group which needs to be switched frequently by the media client, such as an Internet Protocol Television (IPTV), the media client adopting the RAMS solution can switch channels rapidly. Thereby the waiting time for the play of the videos is reduced, and the user experience is improved.
FIG. 1 is a structured flowchart of RAMS system which provides RAMS service in related art. As shown in FIG. 1, the streaming media multicast system comprises: a multicast server 101, a Retransmission Server (RS) 102, a multicast router 103 and a media client 104. The media client is a RTP Receiver (RR). In the RAMS solution, the RS stores all the data sent by the multicast server, performs control signalling communication with the media client, and sends the stored multicast media stream data to the media client by using the unicast and rapid-send mode. The multicast server is for transmitting the media stream to the multicast address in a multicast way. The multicast router is for receiving the media stream of the multicast server, transmitting the media stream to the RS and the media client in a multicast way, forwarding the unicast media stream and the control signalling between the RS and the media client, and processing Source Filtering Group Management Protocol (SFGMP) information. The media client is for receiving and playing the multicast media stream and the media stream data sent from the unicast server in the unicast and rapid-send mode, performing control signalling communication with the unicast server, and sending the SFGMP information to the multicast router. The RS rapidly sends Reference Information (RI) to the RR through the unicast way according to the request of the RR at the media client. The RR analyzes the random access point of the streaming media videos from the received RI and plays it to a user. In the RAMS solution, as the random access point of the RR which joins in the multicast group for the first time is obtained from the RI rapidly sent from the RS rather than obtained from the multicast address, the delay time for playing the pictures for the first time by the media client is no longer affected by the length of the transmission period of the video stream random access point. Therefore, as the RAMS adopts the way of increasing the time interval between the random access points, the coding, decoding and transmission efficiencies are improved.
However, in accordance with the RAMS method, the random access point rapidly sent from the RS to the RR is generally the latest random access point closest to the RR request time. Therefore, different clients after joining in the same multicast group at different time may receive different video random access points sent from the RS. As a result, the videos played by different clients at the same moment are different. Furthermore, the play delay between clients increases along with the gradually increased time interval between the adjacent random access points of the RAMS. In the stream media multicast application, such as the IPTV, video conferences, remote education and the like, excessive delay between the media clients results in a reduction of the user experience, even wrong transfer of information to affect the normal service use of users.